This invention relates generally to an improvement in an electrostatic reproduction machine, but more particularly to an improved corona generating device for such a machine.
In the practice of xerography as described in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface, and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern or electrostatic latent image in conformity with the configuration of the original pattern.
The latent electrostatic image is developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited, and where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the image of the original being produced. The powder image is subsequentily transferred to a sheet of paper or other transfer member, and suitably affixed thereto to form a permanent copy.
The latest concept for electrostatic reproduction, machines utilizes high speed flash exposure of the document, and a moving photoconductive material in the form of an endless belt which is continuously charged. Additionally, such reproduction machines are provided with a developing system which supplies toner particles in relatively large quantities for solid area coverage, such as a magnetic brush developing apparatus. Thus, after the belt passes the magnetic brush assembly, for example, a xerographic powder image is formed on the belt which corresponds to the electrostatic latent image. This powder image is then transferred to a support surface (e.g., a sheet of paper) to which it is fused by a fusing assembly whereby the powder image is caused to adhere to the support surface permanently.
The latest electrostatic reproduction machines are high speed machines which print copies at a rate substantially in excess of any previous electrostatic reproduction machines, and are intended to compete with other types of printing machines, e.g., offset printing machines. Because of this, it is desired that the quality of the copies made, be extremely high. Important to high quality copies are effective corona generating devices. Numerous corona discharge devices are used in such high speed machines. For example, a corona discharge device is used to initially place a charge on the photoreceptor prior to exposure. Corona generating devices are also used prior to the transfer operation to place an appropriate charge on the background so as to prevent or minimize the transfer of background particles. In some machines, corona generating devices are also used to effect the transfer operation. After the transfer operation, corona generating devices are also used to (1) detack the transfer member from the photoreceptor, and (2) to place an appropriate charge on the photoreceptor so as to loosen any residual toner on the photoreceptor so that it may be more easily removed with a brush cleaning apparatus. Thus, as can be seen, corona generating devices are very important to the proper operation of such machines. Many corona generating devices have or must have conductive shields; to keep such corona generating devices operating most effectively, it is highly important that toner be efficiently removed at periodic intervals, because toner accumulations on the shield affect the operation since the shield completes a circuit to the power supply and toner is a dielectric.
In the prior art, various arrangements have been used to remove dirt from such corona generating devices. One method is to pass a high current through the corona wire to burn off any dirt on the wire. While this may remove dirt on the wire, it does not remove the dirt from the shield. Another method has been to use a cleaning pad which contacts both the wire and the shield, the pad generally being made of a porous foam material, e.g., foam urethane. It has been found that pads made of these foam materials do not effectively remove the dirt from the shield and the wire, but merely absorb the dirt and transfer it from one location on the wire or shield to another. Consequently, what is needed is an effective cleaner for a corona generating device which will at least remove substantially all of the dirt on both the shield and the wire.
It is also highly desirable that power supplies designed for various corona generating devices be efficiently used. In a corona generating device having a conductive shield, the total current is equal to the shield current (current between corona wire and shield) plus the plate current (current between corona wire and photoreceptor). Although some corona generating devices may utilize shields which are constructed entirely of a dielectric material, a corona generating device utilized for initially charging a photoreceptor must have a conductive shield to operate properly; the shield is included in the complete circuit of the power supply. In such a device, it is desirable that the plate current be as high as possible (approximately 30% of the total current). One way of increasing the plate current and decreasing the shield current is to construct the shield so that the interior thereof opposite the photoreceptor has a dielectric surface that will increase the plate current component and decrease the shield current component by directing some of the upwardly directed corona emissions downwardly toward the photoreceptor. Naturally, this complicates the manufacturing of the shield and increases the cost of the shield.